The present invention is related to power-on detect circuits, and more particularly, to a fast power-on detect circuit suitable for use in nonvolatile memory and other integrated circuits.
There are accurate power detectors based on the bandgap reference, but they are slow because of the slow response of the operational amplifiers inside the bandgap reference. Thus, this kind of power detector cannot meet most speed requirements. There are timing based power detectors based on RC delay or based on CMOS threshold voltage, but they are very poor in the accuracy of the trip-points. Trying to combine these types of power detectors to achieve both high speed and high accuracy is difficult and unreliable because that involves coordinating between voltage levels and power-up and power-down speeds, and the power-up and power-down profiles are essentially uncertain in the real world.
Power-on detect or power-on reset circuits are used to reset a system to a predetermined state at power-up and power-down. Having a reliable power-on reset circuit is critically important if an incorrect initial state of the circuit in a nonvolatile memory may accidentally cause an access to the memory at a low supply voltage and result in data loss. Thus, the power-on reset circuit must reliably reset the circuit and block any access to the memory when the supply voltage is lower than the minimal safe level and release the gate for access when the supply is high enough. Therefore, a reliable power-on reset circuit with a small variation on its trip-point across the process and temperature corners is always desired for this kind of application.
There are basically three types of power-on reset circuits: RC-based, CMOS-threshold-based, and bandgap-based power-on reset circuits. The first two types have fast responses, but large variations on the trip-points across temperature and process corners. In contrast to the first two types of power-on reset circuits, the bandgap-based power-on reset circuits have narrow variations, but their responses are slow and cannot meet the speed requirements for most integrated circuit applications. Attempting to combine these various types of power detectors to achieve both high speed and high accuracy is difficult and unreliable because that involves coordinating voltage levels and power-up and power-down speeds, and the power-up and power-down profiles are essentially uncertain in the real world.
Referring now to FIG. 1, a prior art bandgap-based power-on reset circuit 100 is shown therein. Bandgap reference block 102 generates a reference voltage Vref. Vref is compared with a voltage V0, which is proportional to power supply VDD and is obtained by scaling down VDD through a resistor divider Ra/Rb. The output PORB of the comparator 104 is a logic zero when V0 is lower than Vref and is a logic one (in this case VDD) otherwise.
A typical bandgap reference block 200 is shown in FIG. 2. Resistors R1, R2, and R3, and PNP transistors Q1, Q2, and Q3 form a temperature compensation structure known in the art. Operational amplifier 204 forces input voltages V1 and V2 to be equal by adjusting the output voltage Vref. When the sizes of the resistors R1, R2, and R3, and PNP transistors Q1, Q2, and Q3 are appropriately selected, the output Vref is equal to two times the silicon bandgap voltage and has a zero temperature coefficient at a desired temperature point. In other words, Vref is equal to two times the bandgap voltage when V1 is equal to V2, which is realized by using operational amplifier 204. As mentioned above, the long settling time of the operational amplifier limits the power-up speed. Thus, power-on reset circuits such as circuit 200 are best suited for use in slow power-up and slow power-down applications. These kinds of circuits have good temperature compensation and provide accurate trip-points, but they are not applicable for fast power-up due to long settle time of the operational amplifiers.
What is desired, therefore, is a power-on reset circuit that overcomes the limitations of the prior art and is suitable for use in fast response applications, yet maintains an accurate trip-point.